If people collectively opt to take some risk, then ethically that's fine. Your car and CO2 emission examples both include reciprocation: everyone benefits and takes risk more or less equally. The problem comes when you participate in putting others at risk when they don't want to reciprocate.

For example, what if 90% of people on a flight would prefer for the 10% of unvaccinated-by-choice people not to be present on the flight at all? They would presumably be fine if the airline put on special "unvaccinated" flights for unvaccinated-by-choice people - except that probably wouldn't be viable as a business model. Since there isn't reciprocation, why should the vaccinated people be forced to be put at risk by you?

What if, instead of flights, we were talking about access to your local grocery store?